In medical cardiac diagnostics there is a need to detect and characterize plaques, lesions and other vascular pathologies and provide information to enable cardiologists to provide adequate therapy. Vulnerable plaques are a specific type of plaque that grows inside the artery and traps lipid within the arterial wall. Due to a natural defense mechanism in the body, such as the effect of macrophage, the thickness of the plaque can be eroded, and when it is down to about 65 microns or less, it is prone to rupture. This rupture releases the lipid into the blood stream and causes a thrombosis. Vulnerable plaque is a leading cause of death by sudden cardiac arrest. The resolution of state of the art technologies, such as MRI and IVUS, is limited to about 150 microns and does not enable measurements down to the critical thickness. The optical interferometric technique known as LCI (Low Coherence Interferometry) provides axial resolution capabilities approximately equal to the coherence length (which is about 10-20 microns with present-day superluminescent diode broadband sources) and therefore is suitable for the detection of vulnerable plaques. One way to construct an LCI instrument is to design an all-fiber interferometer and use the probing fiber component as part of a catheter. However, since the light in the fiber propagates along the fiber axis, some means is required to deflect the probing light toward the arterial wall.
It is also desirable to probe several points around the circumference of an artery at the same time. By probing several points at the same time and pulling back the fiber along the length of the artery, one can examine a length of several centimeters of artery in a short period of time. FIGS. 1a and 1b for example, illustrate two configurations 1 for six fibers inside a one-French guide wire. The guide wire 12 is a flexible hollow tube commonly used in cardiac interventions using catheters that has an inner diameter of less than about 0.3 mm and an outside diameter of about 0.35 mm. Commercial corning SM (Single-Mode) fibers are available in 125-micron clad diameter and in about 80-micron clad diameter. The inner diameter of the guide wire is too small to accommodate six 125-microns diameter fibers, but it may accommodate six 80-micron fibers 10 of diameter 80-microns or less. Thus, while one may work with a smaller number of fibers or a guide wire larger than one-French, a preferred embodiment may be to use six fibers 10 at a time using a SM fiber having an 80-micron clad diameter. FIG. 1a illustrates a configuration with six fibers 10 around a center wire 16. FIG. 1b illustrates a configuration with a hollow central area 18 formed by the internal circumference of the six fibers 10.
Light being propagated through the optical fibers may then be deflected out of the guidewire as illustrated by the arrows 14. One manner in which to deflect the light by, for example, 90° is to grind and polish the fiber tip at 45° and coat the angled surface with a mirror. The resulting transmission through the cylindrical surface of the fiber clad, however, introduces astigmatism in the beam profile. For example, it transforms the beam from a Gaussian shape with a circular cross section in the fiber to one with a highly elliptical cross section. This causes it to spread out in one direction and reduces the backscattered light from the targeted direction, reducing the LCI signal by, for example, about 10 dB or more. Without the astigmatism, the LCI signal is some six orders of magnitude below the incident light from the fiber. The extra loss is not acceptable.
The amount of reflection from the exit plane that is guided back to the detection system is also an important issue. With a high-power light source, it should be reduced as much as possible, preferably less than about −65 dB below the probing light level, in order to keep the so-called RIN (Relative Intensity Noise) below the optical shot noise. One approach to solve both the astigmatism and reflection issues may be to use two components, as illustrated in FIG. 2. This design includes a two component probe; a fiber probe 10 and a separate mirror 24 deposited on a section of similar fiber 22 ground and polished at, for example, 45°. Self alignment may be provided by mounting the two components in a grooved ferrule acting as an optical stage 26. Low fiber tip reflection may be obtained, for example, by cleaving (or cleaving, grinding and polishing) the fiber tip at an angle 28 as shown in FIG. 2, such that the reflected light is away from the fiber core. As an illustration, at normal incidence from a fiber-to-water interface, the reflection back into the fiber core is about −24 dB. With a single-layer AR (anti-reflection) coating on the fiber tip 28, it is decreased to about −35 dB. With an angle, most of the reflected light is away from the fiber core, and the amount of reflected light that is captured by the fiber core (the effective reflection) is below about −65 dB, even without AR coating. The back of the mirrored section 24 may be either ground at an angle or frosted to prevent partially transmitted light from being reflected back into the fiber. While a configuration of the ferrule 26 having a diameter of about 1.5 mm and length of about 3.5 mm preserves the beam profile, it is not suitable for placing several fibers inside a one-French tube.
Accordingly, there is a need for a system and method that prevents astigmatism and minimizes reflection in a fiber probe that can be used, for example, with multiple fibers in guide wire configurations with LCI.